1. Field of the Invention
The present invention relates to a hot wire air flow meter and, more particularly, to a hot wire air flow meter for detecting an amount of intake air of an internal combustion engine.
2. Description of the Prior Art
It has previously been proposed to detect the amount of intake air being introduced to an internal combustion engine using a hot wire air flow meter having a bobbin. (Japanese Patent Laid-Open No. 31412/1984).
The hot wire air flow meter having a bobbin includes a bobbin made of ceramics etc, and a platinum wire forming an exothermic resistor wound in a coil form around the bobbin. A glass material is overcoated on the coiled platinum wire and lead wires are connected to the coiled platinum wire. A supporting member supports the bobbin and the glass coating member.
Quantities of heat for heating the bobbin of the hot wire air flow meter itself, which are also transmitted to the supporting members via the bobbin and the lead wires, cannot be neglected. For the existance of those quantities of heat, the transient response characteristics versus the fluctuation of the amount of air flow has a tendency to a delay. Because of the delay of the transient response characteristics, there is the disadvantage that a surging phenomenon occurs in an automobile under conditions of rapid acceleration and/or rapid deceleration of the automobile.
A hot wire air flow meter is installed generally within a bypass air passage, which bypasses the main air passage of the internal combustion engine. The hot wire air flow meter disposed within the bypass air passage has excellent anti-pulsation characteristics or accurate detection of mean flow of pulsation flow and also anti-backfire characteristics or high shock-resistance against backfire phenomenon.
Besides, it has been known that a platinum wire of an air flow meter may be stretched across a main air passage to form a loop. The platinum wire as the temperature sensitive resistor is held within a ring by spacers in the form of a loop. U.S. Pat. No. 3,824,966).
However, if the air flow meter having such a straight loop-formed platinum wire as shown in the above referenced U.S. Patent should be used in the bypass air passage, it would cause the following faults. Since the space of the bypass air passage is comparatively smaller than that of the main air passage, the length of the platinum wire in the form of a loop would be shorter and limited to a predetermined length. Because of such a shortness of the straight looped-formed platinum wire, the electric resistance value of the air flow meter will be relatively small, and so that output characteristics of the air flow meter may vary widely.